Fix ti.start_date is set even in pre-running state by ti.set_state

[kzosabe]

related: https://github.com/apache/airflow/issues/38632 and https://github.com/apache/airflow/pull/34771#discussion_r1542257954

From the discussion of above PR, ti.start_date is normally set when ti transitions to the RUNNING state. But the current ti.set_state method has set start_date even though the state is prior to RUNNING.

This creates an unstable situation where start_date is present in SCHEDULED and QUEUED ti generated by backfilling, but not in ti that is not.

This is an unnatural behavior for ti's state transitions, so we try to fix it.

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